The present invention relates to an improved fluid flow control device and a compression member therefor
As the technology of semiconductor manufacture, as well as the continuing requirement for higher purity of methods and materials in the scientific, chemical and medical fields matures, attention is being given to the elimination of tiny spaces in fluid flow control devices such as pressure regulators, valves and differential pressure control devices, which can entrap particles and vapors. Such entrapment areas are known as "virtual leaks".
Seals such as valve seats in fluid flow control devices must be held tightly in position against high pressures. In the past, threaded connections have been used to hold or clamp such seals in their designed positions. However, it has been recognized that threaded means can be a source of virtual leaks. Thus, there is a need for an "internally threadless" fluid flow control device in order to insure high purity.
An object of the present invention is to provide an improved fluid flow control device which avoids the aforementioned problem of virtual leaks due to the presence of threaded means. More particularly, an object of the present invention is to provide an improved fluid flow control device wherein a seal is held or clamped in its designed position without the use of internal threaded means.
A further object of the present invention is to provide an improved fluid flow control device which is internally threadless and at the same time is safe; predictable (in engineering terms); unaffected by stressing of the body or cap, heat, cold, vibration or shock; offers clean polished surfaces with low drag to the gas or liquid stream; is designed for a minimum number of parts; offers an absolute minimum of surfaces and shapes that provide "virtual leaks"; and which is designed for minimum assembly time with no possibility of errors.
These and other objects of the invention are attained by the improved fluid flow control device and compression member therefor according to the present invention. The compression member is an engineered structural member, of suitable size, which secures a replaceable seal such as a valve seat in a fluid flow control device such as a pressure regulator or a valve against high forward or reverse pressures. The compression member transmits or produces a concentrated central force on the seal from a diametrically larger force or position.
According to the disclosed embodiment, the larger diameter force or position is transmitted to the compression member through a diaphragm of the fluid flow control device. The diaphragm isolates the compression member and the associated seal from the outside. Since the seal, and the compression member, are generally in the process gas or liquid stream, such sealing off from the outside is important for safety as well as conservation of process materials.
The compression member of the invention acts as a unit load multiplier in the illustrated embodiment producing a relatively high pressure on a small seal with a substantial axial loading being applied to the outer edges of the compression member. The compression member localizes the force on the clamping area acting on the seal. When the force on the compression member is applied with suitable angularity as in the disclosed embodiment, the compression member tensions the diaphragm radially, insuring that it is tight and flat in its unloaded condition.
According to a further feature of the invention, the compression member compresses the seal until a physical stop between the compression member and the body of the device is reached. This results in a fixed preload on the seal, which must be exceeded before a back-pressure can lift the seal and cause a leak through the fluid flow control device. The compression member is designed to produce a predictable preload, which is translated into a back pressure rating for the device.
Since the seal is generally a flat washer-like disc, its axial dimension and the axial dimension of the cavity in the device into which it is clamped can be held to close tolerances with conventional machinery. Thus, the preload established by the geometry of the compression member and the device can be held to an acceptable tolerance in psi without the use of special machine processes.
The compression member is preferably configured as a saucer-like device of an appropriate diameter shaped as a constant stress cantilever of resolution. More particularly, experiments by applicant have shown that the conical shape of the preferred embodiment can be made into a constant stress member with a predictable spring rate which is linear over a sufficiently long range for the necessary preloading in the fluid flow control device. While linearity is not necessary to the functioning of the device it does make the tolerancing of the parts easier.
The compression member is loaded in the fluid flow control device with a strong threaded cap which is screwed down tight against a machine stop so as to preload the compression member to a fixed deflection, with the threads being located outside the fluid passage means of the fluid being regulated or valved with the device. The compression member can yield plastically to some extent during installation as long as the residual force on the seal is sufficient for the intended purpose.
In the preferred embodiment, the compression member comprises a first surface centrally located on one side of the member for applying the concentrated central force and a diametrically larger second surface on a second side of the member opposite the first side, at least a radially outer portion of the second surface is elevated with respect to a radially inner portion thereof to provide a contact surface upon which the diametrically larger force is applied. Fluid passage means extends through the compression member along center axis thereof from the first surface to the second surface. The compression member also includes a portion thereof formed with an increased height as a stop means adjacent the first surface for limiting the movement of the first surface in its application of the concentrated central force.
The height of the second surface of the compression member is preferably progressively increased in a radially outward direction of the compression member. According to the preferred form of the invention, the second surface tapers upwardly at an angle of between 60.degree. and 20.degree. with the second surface of the compression member being defined at least in part by an outwardly protruding annular arm of the compression member which tapers radially outwardly and upwardly. The thickness of the arm is progressively reduced in a radially outward direction and the arm is joined to a central portion of the compression member by way of an undercut groove so that the arm is uniformly stressed along the length thereof in response to the application of the diametrically larger force to the contact surface thereof. This enables the arm to have spring rate under the application of the diametrically larger force which is at least approximately linear over a bending displacement of the radially outer end of the arm of at least 0.010 inch.
The fluid flow control device of the invention comprises the compression member and compressible seal means. The first surface of the compression member transmits the concentrated central force to the seal means of the device in response to the application of the diametrically larger force to the contact surface of the device to thereby clamp the seal in an internally threadless manner so as to avoid virtual leaks. The seal is a valve seat which cooperates with a valve of the device for controlling the flow of a fluid through the device according to the disclosed embodiment. The device further includes means for applying the diametrically larger force of the contact surface of the compression member. The means for applying the diametrically larger force comprises a flexible diaphragm adjacent the compression member through which the larger force is transmitted to the compression member. The fluid flow control device ca be a pressure regulator, valve, differential pressure control device or other fluid flow control device. A body cavity of the device is configured such that the seal as well as the compression member can be dropped in place in appropriate order, the diaphragm added and the clamping cap or ring tightened as various surfaces interlock to guide the part into their designed position. This self centering speeds assembly and reduces the possibility of mistakes.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings, which show, for purposes of illustration only, one preferred embodiment in accordance with the present invention.